The present applicants are the proprietors of a series of inventions in this field. Australian patent application no. AU-B65176/80 (53l,250) describes a hot uniaxial pressing process including embodiments in which a canister having a generally cylindrical wall of bellows-like formation is used to contain the supply material to be pressed and while pressure is maintained by a hydraulic press, heating is applied and the synthetic rock product is formed as the bellows-like canister is axially compressed.
A further patent number AU-72825/82 (524,883) describes a development of the hot uniaxial pressing in which the pressing is conducted in an upward direction against a fixed top abutment in the press.
The art referred to in the above specifications includes prior art of The Australian National University describing the formation of synthetic rock from selected phases and suitable for the immobilisation of radioactive waste.
Typically, synthetic rock precursor is in the form of a fine powder and high-level radioactive waste is a liquid which must be impregnated into the powder in the active cell and pressing must also take place in an active cell. Extremely reliable mechanical handling methods and equipment are required since it is desired for the equipment to operate for tens of years with servicing and repairs conducted only through remote manipulators. In the pressing step, temperatures typically in the range 1050.degree. C. to 1260.degree. C. are experienced with pressures in the range 5 to 30 MPa, pressure being maintained for several hours. On a pilot plant scale, the compressed bellows-like canister incorporating the synthetic rock will have dimensions of the order of 30 cm diameter and 20 cm height and it is very important to achieve extremely high reliability in the pressing stage. For example, it is not acceptable to have a process in which the bellows-like canister might very occasionally split or rupture during the pressing stage since otherwise serious contamination problems would arise. Furthermore, the bellows-like canister must compress in a predictable axial manner with no substantial outward deformation or lateral slippage. The compression must be uniform to ensure proper formation of the synthetic rock structure and, furthermore, usually it will be required to place the canister in some outer container such as a steel cylinder for subsequent safe storage, transportation and disposal.
Active cells are very expensive and therefore a further requirement is that the equipment used in the process is as compact as possible, in addition to being as simple as possible to facilitate servicing; the apparatus used should minimise the capital cost involved in the entire process.
It has been found that a typical synthetic rock precursor powder impregnated with high level waste will be pourable to a packing density of about 19% of the theoretical maximum density whereas after the high temperature pressing step, a density of about 99% of the theoretical maximum density will be achieved. It has been found that such a product is extremely leach resistant.
The present invention is directed to new and useful developments which take into account effectively the general requirements summarised above for synthetic rock processes.